In a CDMA (Code Division Multiple Access) mobile communication system, a plurality of mobile stations use the same frequency band. Usually, the mobile stations each have a different distance from a radio base station. Also, for a mobile station, the radio waves of a radio link from another mobile station cause interference. Therefore, it is preferable for the entire system that the transmission power of each apparatus is minimized within limits in which a predetermined communication quality can be ensured. For this reason, in the CDMA mobile communication system, transmission power for radio base stations and mobile stations is controlled for each mobile station by Inner Loop Power Control (for example, refer to 3PP TS 25.214 V3.7.0, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; “Physical layer Procedures (FDD)”, June 2001, pp. 12-20).
In the CDMA mobile communication system, soft handover is used for switching radio base stations. In soft handover, one mobile station can connect with a plurality of radio base stations. With this system, it is possible to switch radio base stations without disconnection. Also, soft handover contributes to maintaining communication quality by due to the effects of site diversity.
Referring to FIG. 1, in a conventional CDMA mobile communication system, mobile station 93 connects with both radio base station 91 and radio base station 92 by radio links through soft handover. Radio links from radio base stations 91, 92 to mobile station 93 are DL (Down Link). And, radio links from mobile station 93 to radio base stations 91, 92 are UL (Up Link).
Explanations will be given of transmission power control with reference to FIG. 1.
A UL target receiving level is given to radio base stations 91, 92, for example, from a base station control apparatus not shown. Radio base stations 91, 92 control transmission power from mobile stations 93 to ULs so that UL signal receiving levels from mobile stations 93 are close to the UL target receiving level. Incidentally, the UL receiving level and the UL target receiving level are indicated by SIR (Signal to Interference Ratio).
Specifically, radio base stations 91, 92 receive UL signals from mobile station 93 and measure the UL receiving levels. Then, radio base stations 91, 92 compare the UL receiving levels with the UL target receiving level. Subsequently, radio base stations 91, 92 instruct mobile stations 93 to raise transmission power for ULs when the UL receiving levels are smaller than the UL target receiving level. Also, radio base stations 91, 92 instruct mobile station 93 to lower transmission power for the ULs when the UL receiving levels are larger than the UL target receiving levels.
Further, radio base stations 91, 92 change transmission power for DLs in accordance with instructions from mobile station 93. Specifically, radio base stations 91, 92 raise transmission power for DLs when mobile station 93 instructs radio base stations 91, 92 to raise transmission power. Radio base stations 91, 92 lower transmission power for the DLs when mobile station 93 instructs radio base stations 91, 92 to lower transmission power.
Mobile station 93 controls transmission power from radio base stations 91, 92 to DLs so that DL signal receiving levels from radio base stations 91, 92 are close to the predetermined DL target receiving level. Also, the DL receiving level and the predetermined UL receiving level are indicated by SIR or the like.
Specifically, mobile station 93 receives DL signals from radio base stations 91, 92 to synthesize or select the DL signals, and measures the DL receiving levels. Then, mobile station 93 compares the DL receiving levels with the DL target receiving level. Subsequently, mobile station 93 instructs radio base stations 91, 92 to raise the transmission power for DLs when the DL receiving levels are smaller than the DL target receiving level. Also, mobile station 93 instructs radio base stations 91, 92 to lower the transmission power for DLs when the DL receiving levels are larger than the DL target receiving levels.
Further, mobile station 93 changes transmission power for ULs in accordance with instructions from radio base stations 91, 92. Specifically, mobile station 93 raises transmission power for DLs when both radio base stations 91, 92 instruct mobile station 93 to raise transmission power. Mobile station 93 lowers transmission power for ULs when at least one of radio base stations 91, 92 instructs mobile station 93 to lower transmission power.